How low can you go? T-stats that is..

[mrjulian416]

With the stock coolant thermostat of 190F, there have been discussions on this forum about what that means for transmission fluid temps.  With water temps that high there is only so much the transmission cooler can do.  I'm now seeing easily available, lower temp thermostats.

 

Even Autozone now offers 180F, 174F and 160F options for the L8T.  160F seems pretty low.  But how low could you go without causing unintended problems? 

 

Years ago when I was an engineer for a diesel engine manufacturer, our concerns with "too low" water temps was fuel dilution of the oil, excess soot generation, increased emissions from poor combustion.  Other than soot generation, are they the same concerns with gas engines?

 

If you wanted to cool things off a bit, 180? or 174?, or .....?

[bruceb58]

The ECU is programmed to expect 190°F.

[CamGTP]

Good chance you'd set a check engine light for lower temp thermostats, there is a code for coolant undertemp.

 

Honestly 190F is about the perfect temps for coolant, though the engine will run closer to 200F degrees most of the time in warmer weather. That's still less than the L83/L86 trucks that came with 207F degree factory thermostats.

 

With the DI engines I guess technically fuel in the oil is still possible if the engine can't reach proper temp. The hotter the coolant temp, the cleaner the engine should run though. If OEM's could run engines at 250F, they would already be doing it because it would be more efficient, but that is just not possible. Rubber, gaskets and seals won't hold up over time to those temps.

[lundimpact1775]

Just swap the trans thermostat. It's totally separate and unrelated to the engine thermostat. 

 

My engine runs exactly as it was intended, my trans runs cooler by my choice. 6L80 and 6L890 prior to my2014 never had a tstat on the trans fluid and ran full cooling always.

[No F-bdy Bs]

I don't see this as an issue assuming you're talking about the 10 speed. They run very cool even when towing. 

[newdude]

On 10/3/2025 at 8:47 AM, mrjulian416 said:

With the stock coolant thermostat of 190F, there have been discussions on this forum about what that means for transmission fluid temps.  With water temps that high there is only so much the transmission cooler can do.  I'm now seeing easily available, lower temp thermostats.

 

Even Autozone now offers 180F, 174F and 160F options for the L8T.  160F seems pretty low.  But how low could you go without causing unintended problems? 

 

Years ago when I was an engineer for a diesel engine manufacturer, our concerns with "too low" water temps was fuel dilution of the oil, excess soot generation, increased emissions from poor combustion.  Other than soot generation, are they the same concerns with gas engines?

 

If you wanted to cool things off a bit, 180? or 174?, or .....?

 

 

If transmission temps are your concern, 2020 and some 2021 HDs have a 194F transmission bypass valve and have a lower option one that can be installed.  P/N 85547828.  The updated one is full open at 158F.

 

Changing the engine thermostat would be pointless.  

[Pryme]

On 12/16/2025 at 9:08 AM, newdude said:

 

 

If transmission temps are your concern, 2020 and some 2021 HDs have a 194F transmission bypass valve and have a lower option one that can be installed.  P/N 85547828.  The updated one is full open at 158F.

 

Changing the engine thermostat would be pointless.  

Yeah unloaded mine seems to hang out around 151-155 crusin down the road. 

[Black02Silverado]

My 10sp runs 145℉-165℉. The engine temp gauge sits a good bit below the 210℉ line as well.  Now our 2016 Suburban sits right at 210℉ 100%, never moves once it gets to that temp.